1. Field of the Invention
The present invention relates to a photographing light quantity controller for an endoscope, which is used to control the quantity of illuminating light when a photograph is to be taken through the endoscope.
Endoscopes are generally designed to be capable of not only observing the inside of a hollow organ in the patient's body but also taking a photograph of it.
When a photograph is to be taken through an endoscope, the brightness (luminous flux) of illuminating light that is supplied to the endoscope is set to a maximum level and, in this state, the output of a light-receiving element that receives the reflected light from an object is integrated; when the integral output voltage reaches a reference voltage, a mechanical shutter that is provided in an illuminating light path within a light source device is closed, thereby controlling the exposure time.
However, such a mechanical shutter takes some time from the instant it receives a signal for closing until it has been completely closed, and the exposure therefore becomes correspondingly excessive. The exposure time .DELTA.T that corresponds to an excess of exposure is constant independently of the length of the overall exposure time on each particular occasion. Accordingly, when the overall exposure time T is relatively short, that is, when the object is relatively bright as in the case of close-up photography, the effect of .DELTA.T becomes significant, resulting in a high degree of over-exposure.
2. Description of the Prior Art
To reduce the degree of over-exposure, a method has heretofore been employed in which the rise of a signal representative of an integral state value which is obtained by integrating the output of a light-receiving element is detected differentially, and when the differential output value is greater than a reference value, the brightness of a light source is lowered to an observational state level to increase the overall exposure time T, thereby reducing the effect of .DELTA.T.
However, even if such an over-exposure control measure is taken, some problems still remain unsolved. For example, the illuminating light flux is set one set to a maximum level at the time of photographing, thus a picture is taken at an extremely short distance to the object, or when a fast film is employed, the correction of the illuminating light flux cannot be effectively made, and over-exposure is unavoidable. In addition, since the brightness of the illuminating light is merely lowered to a level for the observation after the correction, no delicate control can be effected, so that the exposure time may become excessively long, resulting in blurring or other problems.